Internet Engineering Task Force C. Perkins
INTERNET DRAFT IBM
21 October 1995
IP Encapsulation within IP
draft-ietf-mobileip-ip4inip4-01.txt
Status of This Memo
This document is a submission by the Mobile-IP Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the mobile-ip@tadpole.com mailing list.
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Abstract
This document specifies a method by which an IP datagram may
be encapsulated (carried as payload) within an IP datagram.
Encapsulation is suggested as a means to effect "re-addressing"
datagrams (i.e., delivering them to an intermediate destination
other than that specified in the IP destination field) for any of a
variety of reasons, but particularly those useful for adherence to
the mobile-IP specification.
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1. Introduction
This document specifies a method by which an IP datagram may
be encapsulated (carried as payload) within an IP datagram.
Encapsulation is suggested as a means to effect "re-addressing"
datagrams -- that is, delivering them to an intermediate destination
other than that specified in the IP destination field. The process
of encapsulation and decapsulation a datagram is frequently referred
to as "tunneling" the datagram, and the encapsulator and decapsulator
are then considered to be the the "endpoints" of the tunnel.
In the most general encapsulation case we have
source ----> encapsulator --------> decapsulator ----> destination
with these being separate machines. There may in general be multiple
source-destination pairs using the same tunnel.
2. Motivation
The mobile-IP working group has specified the use of encapsulation as
a way to deliver packets from a mobile host's "home network" to an
agent which can deliver packets to the mobile host by conventional
means [4]. The use of encapsulation may also be desirable whenever
the source (or an intermediate router) of an IP datagram must
influence the route by which a datagram is to be delivered to
its ultimate destination. Other possible applications include
preferential billing, choice of routes with selected security
attributes, and general policy routing.
It is generally true that encapsulation and source routing techniques
can both be used to re-address datagrams whenever that is necessary,
but there are several technical reasons to prefer encapsulation:
- There are unsolved security problems associated with the use of
source routing.
- Current internet routers exhibit performance problems when
forwarding packets which use the IP source routing option.
- Too many internet hosts process source routing options
incorrectly.
- Firewalls may exclude source-routed packets.
- Insertion of an IP source route option may complicate the
processing of authentication information by the source and/or
destination of a datagram, depending on how the authentication is
specified to be performed.
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- It is considered impolite for intermediate routers to make
modifications to the packets which they did not originate.
These technical advantages must be weighed against the disadvantages
posed by the use of encapsulation:
- Encapsulated packets typically are longer than source routed
packets.
- Encapsulation cannot be used unless it is known in advance that
the tunnel endpoint for the encapsulated datagram can decapsulate
the packet.
Since the majority of internet hosts today do not perform well
when IP loose source route options are used, the second technical
disadvantage of encapsulation is not as serious as it might seem at
first.
3. IP in IP Encapsulation
An outer IP header is inserted before the datagram's IP header:
+---------------------------+
| Outer IP Header |
+---------------------------+ +---------------------------+
| IP Header | | IP Header |
+---------------------------+ ====> +---------------------------+
| | | |
| IP Payload | | IP Payload |
| | | |
+---------------------------+ +---------------------------+
The format of the IP header is described in RFC 791[9]. The outer
IP header source and destination addresses identify the "endpoints"
of the tunnel. The inner IP header source and destination addresses
identify the sender and recipient of the datagram. The inner IP
header is not changed by the node which encapsulates it, except
to decrement the TTL before delivery. The inner header remains
unchanged during its delivery to the tunnel endpoint. No change
to IP options in the inner header occurs during delivery of the
encapsulated packet through the tunnel.
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3.1. IP Header Fields and Handling
Version
4
IHL
The Internet Header Length measures the length (in bytes) of
the outer IP header exclusive of its payload, but including any
options which the encapsulating agent may insert.
TOS
The type of service is copied from the inner IP header.
Total Length
The length measures the length of the outer IP header along
with its payload, that is to say the inner IP header and the
original datagram.
Identification
Flags
Fragment Offset
These three fields are set in accordance with the procedures
specified in [9]. The "Don't Fragment" bit in the outer IP
header is copied from the corresponding flag in the inner IP
header.
Time to Live
The Time To Live (TTL) field in the outer IP header is set to a
value appropriate for delivery of the encapsulated datagram to
the tunnel endpoint.
Protocol
The protocol field in the outer IP header is set to protocol
number 4 for the encapsulation protocol.
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Header Checksum
The Header Checksum is computed over the length (in bytes) of
the outer IP header exclusive of its payload, but including any
options which the encapsulating endpoint may insert.
Source Address
The IP address of the encapsulating agent, that is, the tunnel
starting point.
Destination Address
The IP address of the decapsulating agent, that is, the tunnel
completion point.
Options
not copied from the inner IP header. However, new options
particular to the path MAY be added. In particular, any
supported flavors of security options of the inner IP header
MAY affect the choice of security options for the tunnel. It
is not expected that there be a one-to-one mapping of such
options to the options or security headers selected for the
tunnel.
The inner TTL is decremented by one. If the resulting TTL is 0,
the datagram is not tunneled. An encapsulating agent MUST NOT
encapsulate a packet with TTL = 0 for delivery to a tunnel endpoint.
The TTL is not changed when decapsulating. If, after decapsulation,
the inner packet has TTL zero, a tunnel endpoint MUST discard the
packet. If the decapsulator forwards the datagram to some network
interface, it will decrement the TTL as a result of doing normal IP
forwarding. See also subsection 4.4.
The encapsulating agent is free to use any existing IP mechanisms
appropriate for delivery of the encapsulated payload to the tunnel
endpoint. In particular, this means that use of IP options and
fragmentation are allowed, unless the "Don't Fragment" bit is set in
the inner IP header. This is required so that hosts employing Path
MTU discovery [7] can obtain the information they seek.
3.2. Routing Failures
Routing loops within a tunnel are particularly dangerous when they
arrive again at the encapsulator. If the IP Source matches any of
its interfaces, an implementation MUST NOT further encapsulate. If
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the IP Source matches the tunnel destination, an implementation
SHOULD NOT further encapsulate. See also subsection 4.4.
4. ICMP messages from within the tunnel
After an encapsulated datagram has been sent, the encapsulating
agent may receive an ICMP [8] message from any intermediate router
within the tunnel, except for the tunnel endpoint. The action taken
by the encapsulating agent depends on the type of ICMP message
received. When the received message contains enough information, the
encapsulating agent may use the incoming message to create a similar
ICMP message, to be sent to the originator of the inner IP datagram.
This process will be referred to as "relaying" the ICMP message to
the source of the original unencapsulated datagram. Relaying an ICMP
message requires that the encapsulator must strip off the outer IP
header which it receives from the sender of the ICMP message. For
cases where the received message does not contain enough data, see
section 5.
4.1. Destination Unreachable (Type 3)
Destination Unreachable messages are handled depending upon their
type. The model suggested here allows the tunnel to "extend" a
network to include non-local (e.g., mobile) hosts. Thus, if the
original destination in the unencapsulated datagram is on the same
network as the encapsulating agent, certain Destination Unreachable
codes may be modified to conform to the suggested model.
Network Unreachable (Code 0)
A Destination Unreachable message may be returned to
the original sender. If the original destination in
the unencapsulated datagram is on the same network as
the encapsulating agent, the newly generated Destination
Unreachable message sent by the encapsulating agent can have
code 1 (Host Unreachable), since presumably the packet arrived
to the correct network and the encapsulating agent is trying to
create the appearance that the original destination is local
even if it's not. Otherwise, the encapsulating agent must
return a Destination Unreachable with code 0 message to the
original sender.
Host Unreachable (Code 1)
The encapsulating agent must relay Host Unreachable messages to
the source of the original unencapsulated datagram.
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Protocol Unreachable (Code 2)
When the encapsulating agent receives a Protocol Unreachable
ICMP message, it should send a Destination Unreachable message
with code 0 or 1 (see the discussion for code 0) to the sender
of the original unencapsulated datagram. Since the original
sender might only rarely use protocol 4, it would be usually be
meaningless to return code 2 to that sender.
Port Unreachable (Code 3)
This code should never be received by the encapsulating
agent, since the outer IP header does not refer to any port
number. It must not be relayed to the source of the original
unencapsulated datagram.
Datagram Too Big (Code 4)
The encapsulating agent must relay Datagram Too Big messages to
the source of the original unencapsulated datagram.
Source Route Failed (Code 5)
This code should be treated by the encapsulating agent
itself. It must not be relayed to the source of the original
unencapsulated datagram.
4.2. Source Quench (Type 4)
The encapsulating agent may relay Source Quench messages to the
source of the original unencapsulated datagram.
4.3. Redirect (Type 5)
The encapsulating agent may act on the Redirect message if it is
possible, but it should not relay the Redirect back to the source of
the datagram which was encapsulated.
4.4. Time Exceeded (Type 11)
ICMP Time Exceeded messages report routing loops within the tunnel
itself. Reception of Time Exceeded messages by the encapsulator
MUST be reported to the originator as Host Unreachable (Type 3 Code
1). Host Unreachable is preferable to Network Unreachable; since the
packet was handled by the encapsulator, and the encapsulator is often
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considered to be on the same network as the destination address in
the original unencapsulated packet, the packet is considered to have
reached the correct network, but not the correct destination host
within that network.
4.5. Parameter Problem (Type 12)
If the parameter problem points to a field copied from the original
unencapsulated datagram, the encapsulating agent may relay the ICMP
message to the source; otherwise, if the problem occurs with an IP
option inserted by the encapsulating agent, then the encapsulating
agent must not relay the ICMP message to the source. Note that an
encapsulating agent following prevalent current practice will never
insert any IP options into the encapsulated datagram, except possibly
for security reasons.
4.6. Other messages
Other ICMP messages are not related to the encapsulation operations
described within this protocol specification, and should be acted on
as specified in [8].
5. Tunnel Management
Unfortunately, ICMP only requires IP routers to return 8 bytes (64
bits) of the datagram beyond the IP header. This is not enough to
include the encapsulated header, so it is not always possible for the
home agent to immediately reflect the ICMP message from the interior
of a tunnel back to the source host.
However, by carefully maintaining "soft state" about its tunnels,
the encapsulating router can return accurate ICMP messages in most
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cases. The router SHOULD maintain at least the following soft state
information about each tunnel:
- MTU of the tunnel (subsection 5.1)
- TTL (path length) of the tunnel
- Reachability of the end of the tunnel
The router uses the ICMP messages it receives from the interior of a
tunnel to update the soft state information for that tunnel. ICMP
errors that could be received from one of the routers along the
tunnel interior include:
- Datagram Too Big
- Time Exceeded
- Destination Unreachable
- Source Quench
When subsequent datagrams arrive that would transit the tunnel,
the router checks the soft state for the tunnel. If the datagram
would violate the state of the tunnel (such as, the TTL is less than
the tunnel TTL) the router sends an ICMP error message back to the
source, but also forwards the datagram into the tunnel.
Using this technique, the ICMP error messages sent by encapsulating
routers will not always match up one-to-one with errors encountered
within the tunnel, but they will accurately reflect the state of the
network.
Tunnel soft state was originally developed for the IP address
encapsulation (IPAE) specification [3].
5.1. Tunnel MTU Discovery
When the Don't Fragment bit is set by the originator and copied
into the outer IP header, the proper MTU of the tunnel will
be learned from ICMP (Type 3 Code 4) "Datagram Too Big" errors
reported to the encapsulator. To support originating hosts
which use this capability, all implementations MUST support Path
MTU Discovery([6, 7]) within their tunnels. In this particular
application there are several advantages:
- As a benefit of Tunnel MTU Discovery, any fragmentation which
occurs because of the size of the encapsulation header is done
only once after encapsulation. This prevents more than one
fragmentation of a single datagram, which improves processing
efficiency of the path routers and tunnel decapsulator.
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- If the source of the unencapsulated packet is doing MTU discovery
then it is desirable for the encapsulator to know the MTU to the
decapsulator. If it doesn't know the MTU then it can transfer
the DF bit to the outer packet; however, if that triggers ICMP
Datagram Too Big from within the tunnel (and hence returned
to the encapsulator) the encapsulator cannot always return a
correct ICMP response to the source unless it has kept state
information about recently sent packets. If the tunnel MTU is
returned to the source by the encapsulator in a Datagram Too Big
message, the MTU that is conveyed SHOULD be the MTU of the tunnel
minus the size of the encapsulating IP header. This will avoid
fragmentation of the original IP datagram by the encapsulator,
something that is otherwise certain to occur.
- If the source is not doing MTU discovery it is still desirable
for the encapsulator to know the MTU to the decapsulator. In
particular it is much better to fragment the inner packet than
to allow the outer packet to be fragmented. Fragmenting the
inner packet can be done without special buffer requirements and
without the need to keep state in the decapsulator. By contrast
if the outer packet is fragmented then the decapsulator needs to
keep state and buffer space on behalf of the destination.
The encapsulator SHOULD in normal circumstances do MTU discovery and
try to send packets with the DF bit set. However there are problems
with this approach. When the source sets the DF bit it can react
quickly to resend the information if it gets a ICMP Datagram Too
Big. When the encapsulator gets a ICMP Datagram Too Big, but the
source had not set the DF bit, then there is nothing sensible that
the encapsulator can do to let the source know. The encapsulator
MAY keep a copy of the sent packet whenever it tries increasing the
MTU - this will allow it to resend the packet fragmented if it gets
a packet too big response. Alternatively the encapsulator MAY be
configured for certain classes of input to not set the DF bit when
the source has not set the DF bit.
5.2. Congestion
Tunnel soft state will collect indications of congestion, such as
an ICMP (Type 4) Source Quench or a Congestion Experienced flag in
datagrams from the decapsulator (tunnel peer). When forwarding
another datagram into the tunnel, it is appropriate to send Source
Quench messages to the originator.
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6. Security Considerations
IP encapsulation potentially reduces the security of the Internet.
For this reason care needs to be taken in the implementation and
deployment.
Assume an organization has good physical control of a secure subset
of its network. Assume that the routers connecting that secure
network do not allow in packets with source addresses belonging to
interfaces on that secure network. In that situation it is possible
to safely deploy protocols within that network which depend on the
source address of packets for authentication purposes.
Networks with physical security can still be used to run protocols
which are convenient, but which have implementation or protocol bugs
which would make them dangerous to use if external sources have
access to the protocol. The external sources can be excluded using
router packet filtering.
IP encapsulation protocols may allow packets to bypass the checks in
the border routers. There are two cases to consider:
- The case where the people controlling the border routers are
trying to protect inner machines from themselves.
- The case where the inner machine is looking after its own
defense.
An uncooperative inner machine cannot be protected by the border
router except by barring all packets to that machine. There is
nothing to stop encapsulated IP coming in to that inner machine
in otherwise harmless packets such as port 53 UDP packets (i.e.,
apparently DNS packets). So there is a strong case for placing the
security controls at the host rather than the router. However, in
situations where the administrative control of the inner machine is
cooperative but lacks thoroughness or competence, security can be
enhanced by also putting protection in the border routers.
6.1. Router Considerations
Routers need to be aware of IP encapsulation protocols so they can
correctly filter incoming packets.
Beyond that it is desirable that filtering be integrated with IP
authentication [1]. In the case of IP encapsulation this can have
2 forms: Encapsulation might be allowed (in some cases) as long
as the encapsulating packets authentically come from an expected
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encapsulator. Alternatively encapsulation might be allowed if the
encapsulated packets have authentication.
The next problem is with packets which are encapsulated and
encrypted [2]. In this case the router can only filter the packet if
it knows the security association. To allow this sort of encryption
in environments where all packets need to be filtered (or at least
accounted for) a mechanism must be in place for the receiving host
to securely communicate the association to the border router. This
might, more rarely, also apply to the association used for outgoing
packets.
6.2. Host Considerations
Receiving IP encapsulation software SHOULD classify incoming packets
and only allow packets fitting one of the following categories:
- The protocol is harmless: source address based authentication is
not needed.
- The packet can be trusted because of trust in the authentically
identified encapsulating host. That authentic identification
could come from physical security plus border router
configuration but is more likely to come from AH authentication.
- The inner packet has AH authentication.
Some or all of this checking could be done in border routers rather
than the receiving host but it is better if border router checks are
used as backup rather than being the only check.
6.3. Using Security Options
The security options of the inner IP header MAY affect the choice of
security options for the encapsulating IP header.
7. Acknowledgements
Parts of sections 3 and 5 were taken from the mobile-IP draft [5].
Good ideas have also been included from RFC 1853 [10]. "Security
Considerations" (section 6) was largely contributed by Bob Smart.
Thanks also to Anders Klemets for finding mistakes and suggesting
many improvements to the draft.
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References
[1] R. Atkinson. IP Authentication Header. RFC 1826, August 1995.
[2] R. Atkinson. IP Encapsulating Security Payload. RFC 1827,
August 1995.
[3] R. Gilligan, E. Nordmark, and B. Hinden. IPAE: The SIPP
Interoperability and Transition Mechanism. Internet Draft --
work in progress, March 1994.
[4] IETF Mobile-IP Working Group. IPv4 Mobility Support.
ietf-draft-mobileip-protocol-12.txt - work in progress,
September 1995.
[5] IETF Mobile-IP Working Group. IPv4 Mobility Support.
ietf-draft-mobileip-protocol-10.txt -- outdated draft, May 1995.
[6] S. Knowles. IESG Advice from Experience with Path MTU
Discovery. RFC 1435, March 1993.
[7] J. Mogul and S. Deering. Path MTU Discovery. RFC 1191,
November 1990.
[8] J. Postel. Internet Control Message Protocol. RFC 792,
September 1981.
[9] J. Postel. Internet Protocol. RFC 791, September 1981.
[10] W. Simpson. IP in IP Tunneling. RFC 1853, October 1995.
Author's Address
Questions about this memo can also be directed to:
Charles Perkins
Room J1-A25
T. J. Watson Research Center
IBM Corporation
30 Saw Mill River Rd.
Hawthorne, NY 10532
Work: +1-914-784-7350
Fax: +1-914-784-7007
E-mail: perk@watson.ibm.com
The working group can be contacted via the current chairs:
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Jim Solomon Tony Li
Motorola, Inc. cisco systems
1301 E. Algonquin Rd. 170 W. Tasman Dr.
Schaumburg, IL 60196 San Jose, CA 95134
Work: +1-708-576-2753 Work: +1-408-526-8186
E-mail: solomon@comm.mot.com E-mail: tli@cisco.com
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